Glow plugs are provided in auto-ignition engine, also referred to as compression ignition engines, to aid in heating the engine during a cold start. In the past glow plugs have been provided in pre-chambers to facilitate heating of the air fuel mixture before entering the combustion chamber. Engines have been designed to move the glow plugs in the pre-chambers to affect the airflow in the pre-chamber. However, pre-chamber engine designs have decreased combustion efficiency when compared to direct injection cylinder designs. Furthermore, the inventors have also recognized a need to vary the size of the combustion chambers in auto-ignition engines based on varying engine operating conditions.